Reduction of arc-tracking in chip on flexible circuit substrates

ABSTRACT

A method of manufacturing a flexible circuit, including forming a first set of traces, forming a second set of traces, and cutting a gap between the first and second set of traces.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. patent application Ser. No.13/314,071, filed on Dec. 7, 2011, titled “Reduction of Arc-Tracking InChip On Flexible Circuit Substrates,” which is incorporated herein inits entirety.

BACKGROUND

Many devices use flexible circuit substrates. Integrated circuits, orchips, mount on the flexible circuit substrate and use the traces in theflexible circuit substrate like a typical circuit substrate similar to aprinted circuit board. The flexible circuit substrate generally consistsof polymers and other relatively carbon rich materials. In someinstances, the carbon of the flexible circuit substrate can become fuelfor an arc event, charring, thermal event, etc.

If an electrical short occurs in a chip mounted on the flexible circuitsubstrate, such as between the negative voltage supply (Vss) and a highvoltage positive supply (Vpp), excessive heat results from the excessivecurrent flow between Vpp and Vss due to the low electrical resistanceshort within the chip. The short within the chip can be created bydifferent factors, including a defective part, electrical over-stress,etc. The high temperature carbonizes the flexible material, and carbonbeing electrically conductive, this forms another electricallyconductive, carbon path outside of chip between the supply traces suchas Vpp and Vss or any other nearby trace that becomes involved, such astraces that become shorted to Vpp or Vss, with carbonized flexiblematerial. This further increases the current flow through the lowerresistance area, in turn generating more heat. More current producesmore heat which carbonizes even more material and the process becomesself-propagating as long as sufficient fuel for generating electricallyconductive carbon, such as the flexible circuit substrate, and currentfrom power supplies remain available.

The arc ‘tracks’ along the current path, carbonizing material along theway. The carbonized material, as in the case of polyimide (Kapton™ is abrand name of a polyimide flexible circuit material), may also releasehydrogen that readily ignites and consumes more of the flex materialuntil the entire Vss and Vpp signal planes are carbonized. This problemhas resulted in certain substrate materials being banned for use incertain industries such as in the aircraft industry. However, usingflexible circuit substrates allows for tighter packaging and smallerdevices, as well as typically lower costs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a printhead jet stack employing a flexiblecircuit substrate.

FIG. 2 shows an embodiment of an integrated circuit on a flexiblecircuit substrate having an arc-break.

FIG. 3 shows an example of a flexible circuit substrate with anarc-break after an induced thermal event.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a block diagram of an ink jet print head 10. One mustunderstand that this provides merely one example of a device that usesintegrated circuits on flexible circuit substrates, or ‘chip-on-flex’(COF). Any device that employs COF architectures may benefit from theembodiments discussed here. No limitation is implied nor should one beinferred from the examples given here to assist with understanding ofthe invention.

The print head 10 may operate in a solid-ink printer in which the inksupply consists of solid ink. A heater within the printer melts the inkand transports it to the print head for printing onto a print substrate.The print head typically consists of a nozzle plate, such as 18, havingan array of holes or apertures through which ink exits the print head.The nozzle plate 18 may be one of several plates formed into a stacksometimes referred to as a jet stack such as 16.

The plates stack together to form channels and reservoirs within thestack to route the ink to chambers adjacent each nozzle. A membrane 15operates in response to an array of transducers 13 to fill the chamberswith ink and dispense them through the apertures. The transducers inturn receive signals from the flexible circuit substrate 12, the signalsin their serial form originating from the printer engine aredeserialized by the integrated circuit 14. This integrated circuit alsoreceives power signals, in this example, both negative supply voltagesignals, referred to here as Vss, and positive signals, referred to hereas Vpp.

A short may occur in the integrated circuit 14 between these two signalsthat produces excessive current flow because of the low resistanceshort. Many flexible circuit substrates consist of polymer materialscontaining carbon and hydrogen. The heat generated from the short cancarbonize the flexible material and carbonized material conducts morecurrent, reducing the resistance even more. This starts a chain reactioncalled “arc-tracking” that continues until it runs out of material orthe current is removed. In the process of arc-tracking, some flexiblematerials such as polyimide releases hydrogen that can readily ignite ina micro-flame, further increasing the temperature and the carbonizedarea.

By stopping the arc-tracking, one can stop the chain reaction and avoidfurther destruction of the material as well as avoiding further a largerthermal event. When the arc-tracking stops, the current flow no longerincreases, and the localized burned area extinguishes itself. Oneembodiment that stops arc-tracking employs an arc-break. The arc-breaksmay take the form of gaps in the material between the two current paths,Vpp and Vss in the current example, coming out of the integratedcircuit. FIG. 2 shows an example of an integrated circuit mounted on aflexible circuit substrate.

In FIG. 2, the integrated circuit 14 resides on a flexible circuitsubstrate 12. The integrated circuit has traces 20 for the negativesupply voltage, and 22 for the positive supply voltage. If a shortoccurs in the integrated circuit 14, the carbonization will startbetween the two sets of traces and the arc-tracking will continue untilit reaches the first arc-break 24 or 30. The first set of arc-breaksconstitutes a primary arc-break as it separates the main high currentcarrying traces.

A secondary arc-break such as 26 or 28 arranged between the firstarc-break and other traces, may also assist in preventing thecarbonization from involving other traces such as logic traces in themiddle, which would result in current flowing from the supply traces tothe logic devices, possibly damaging them. These gaps remove thepossibility of carbon creation between high current carrying traces andother nearby traces, such that current is limited to flow through theoriginal short inside the chip and small area of carbonized materialnear the chip before the arc ‘breaks.’ This creates a very localizedheat that acts as a fuse in that is causes the traces to burn openthereby clearing the short and preventing further current flow.

FIG. 3 shows an example of an experiment employing arc-breaks. Aflexible circuit substrate 12 has an integrated circuit 14. Theexperiment involved causing a short in the integrated circuit betweenthe Vss and Vpp signals in the integrated circuit. Without thearc-breaks, such as 24, the arc would track up the traces such as 20 and22 until the material in the flexible circuit substrate in the generalregion 40 had been consumed. The consumption of this material would haveresulted in a much longer arc-tracking event and much more violentcharring as the hydrogen released would have further ignited.

However, because of the arc-break, the arc contained itself to the area42. While damage still occurred to the flexible circuit substrateindicated by 42, no further destruction occurred.

Other forms of arc-breaks may also provide this function. Instead ofremoving material, one could add a thermal control material in betweenthe traces to stop the arc-tracking. However, additional materials maycomplicate the manufacturing process and the materials would have towork with the other parts of the manufacturing process.

Removing of the material for the arc-break may take many forms. Forexisting flexible circuit substrates, the process may include cutting orotherwise forming the arc-break areas. In one embodiment, the materialin the arc-break area may undergo laser cutting for removal. The lasercutting may occur as part of the cutting of the flexible circuitsubstrate from the tape roll upon which it was formed.

In another embodiment, the flexible circuit substrate may undergoprocessing at formation that removes the material to form the arc-break.The artwork used in forming the flexible circuit substrate may includethe regions to be used as an arc-break or breaks.

While the above discussion began in the context of a chip-on-flexiblecircuit substrate for an ink jet print head, any flexible circuit thathave high current carrying traces can use the embodiments describedhere. Any device that employs a flexible circuit substrate containingcarbon and hydrogen could undergo arc-tracking and would benefit fromthe embodiments here.

It will be appreciated that several of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations, or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

What is claimed is:
 1. A method of manufacturing a flexible circuit,comprising: forming a first set of traces; forming a second set oftraces; and cutting a gap between the first and second set of traces. 2.The method of claim 1, wherein cutting the gap comprises using a laserto cut the gap.
 3. The method of claim 1, wherein cutting the gapcomprises cutting the gap when the circuit substrate is cut from alarger piece of material.
 4. The method of claim 1, wherein cutting thegap comprises forming the gap during formation of the flexible circuit.5. The method of claim 1, wherein forming a first set of tracescomprises forming a first set of traces and connecting the first set oftraces to a negative voltage supply.
 6. The method of claim 1, whereinforming a second set of traces comprises forming a second set of tracesand connecting the second set of traces to a positive voltage supply. 7.The method of claim 1, further comprising forming a second gap betweenthe gap and at least one of the sets of traces.
 8. A method ofmanufacturing a flexible circuit, comprising: forming the flexiblecircuit having at least one region with an arc-break in the flexiblecircuit; forming a first set of traces adjacent the gap on a first sideof the arc-break; and forming a second set of traces adjacent the gap ona side of the arc-break opposite the first side.
 9. The method ofmanufacturing of claim 8, wherein forming the flexible circuit having atleast one region with an arc-break comprises forming the flexiblecircuit with thermal control material to form the arc-break.
 10. Themethod of manufacturing of claim 8, wherein forming the flexible circuithaving at least one region with an arc-break comprises forming theflexible circuit by removing material to form the arc-break.
 11. Amethod of forming a print head, comprising: forming a first set oftraces on a flexible substrate; forming a second set of traces on aflexible substrate; cutting a gap between the first and second set oftraces in the flexible substrate; stacking a set of plates together toform a jet stack; bonding the jet stack to the flexible substrate; andmounting an integrated circuit on the flexible substrate.
 12. The methodof claim 11, wherein cutting the gap comprises using a laser to cut thegap.
 13. The method of claim 11, wherein cutting the gap comprisescutting the gap when the circuit substrate is cut from a larger piece ofmaterial.
 14. The method of claim 11, wherein cutting the gap comprisesforming the gap during formation of the flexible circuit.
 15. The methodof claim 11, wherein forming a first set of traces comprises forming afirst set of traces and connecting the first set of traces to a negativevoltage supply.
 16. The method of claim 11, wherein forming a second setof traces comprises forming a second set of traces and connecting thesecond set of traces to a positive voltage supply.
 17. The method ofclaim 11, further comprising forming a second gap between the gap and atleast one of the sets of traces.